In the fabrication of integrated circuits, a number of processes have been established which involve the application of ion beams onto semiconductor wafers in vacuum. These processes include ion implantation, ion beam milling and reactive ion etching. In each instance, a beam of ions is generated in an ion source and is directed with varying degrees of acceleration toward a target. Ion implantation has become a standard technique for introducing impurities into semiconductor wafers. Impurities are introduced in the bulk of semiconductor wafers by using the momentum of energetic ions as a means of embedding them in the crystalline lattice of the semiconductor material. Ion implantation has also been used for altering the properties of metals and polymers.
The ion source for producing the ion beam includes a chamber with suitable applied electric and magnetic fields to ionize molecules of the desired species. A source material to be ionized is fed to the ion source continuously for ionization to form a continuous beam. The source material may be supplied as a gas or a solid, depending on its chemical and physical properties. When a solid material is used, it is placed in a vaporizer which heats the source material in a crucible to a temperature to produce a controlled amount of vapor of the source material. The vapor is then supplied through a conduit to the ion source for ionization. Various types of ion sources are known in the prior art.
Most prior art vaporizer systems include one crucible containing the source material, an electric heater and a thermal insulator (heat shield) to prevent heat loss and increase the efficiency of heating power. The heater and the thermal insulator are physically attached to the crucible. Other prior art systems utilize multiple crucibles, but the basic construction of each crucible remains the same. That is, each crucible has a separate electric heater physically attached to it, and the electric heaters are individually energized.
Prior art vaporizer systems have numerous disadvantages. The thermal time constant of such systems is relatively slow because of additional thermal masses of the heater and thermal insulation. The initial warm-up time and the time to change from one source material to another is relatively long because of the associated warm-up and cool-down times. Such delays are highly undesirable in a commercial ion implantation system where minimization of downtime is important. In addition, servicing of prior art heaters and crucibles is difficult since the heater and crucible are essentially inseparable. Furthermore, servicing the heater always means handling the crucible which quite often contains poisonous materials. Heater life is affected by the quality of the physical contact with the crucible. Loose contact, which always occurs to some extent, elevates the heater temperature and results in premature heater failure. Multiple crucible designs in accordance with the prior art are complicated and tend to be less reliable as the number of heaters increases. Furthermore, the number of electrical feedthroughs through the vacuum to atmosphere interface increases. All these factors tend to increase the cost of the system and reduce its reliability.
It is a general object of the present invention to provide improved vaporizer systems for ion sources.
It is another object of the present invention to provide a vaporizer system having one or more crucibles and a single radiation source which in the case of multiple crucibles is directed to a selected one of the crucibles.
It is another object of the present invention to provide a vaporizer system having a relatively short thermal time constant for heating and cooling.
It is a further object of the present invention to provide a vaporizer system which is relatively safe and which is easy to service and maintain.
It is yet another object of the present invention to provide a vaporizer system which is low in cost and which is relatively high in reliability.